In recent years, there are demands for laminates (which, in the present invention, include a laminate covered with a metal foil on the top or the top and bottom, namely, a metal-clad laminate) and prepregs for electrical insulation, in which they should have a high stiffness and a high dielectric constant or contrary a low dielectric constant, and be free of halogens, and the laminates and prepregs tend to contain various inorganic fillers in a large amount for meeting such demands. Especially the production of halogen-free laminates or prepregs is carried out for avoiding a so-called dioxin problem that arises when burning materials containing halogenated compounds, and a large amount of an inorganic oxide or hydroxide is frequently added to impart flame retardancy to a laminate or prepreg without using a halogenated flame retardant.
However, when a large amount of inorganic filler is added to the resin system which has conventionally and generally been used, in which an epoxy resin is cured by dicyandiamide, separation between the resin and the filler may occur during lamination, resulting in a non-uniform laminate. This is a characteristic phenomenon of the dicyandiamide curing, which phenomenon is barely observed in other resin systems such that an epoxy resin is cured by other curing agents (a novolak resin, etc.), and this phenomenon is remarkable especially when an epoxy resin having a phenolic novolak skeleton or a cresol novolak skeleton is used in a large amount. Such a non-uniform laminate has not only a problem about appearance but also a problem in that various properties of the laminate are degraded, especially the resistance to soldering heat is markedly reduced. There is an epoxy resin which does not cause the above-mentioned separation between the resin and the filler, but, for obtaining excellent heat resistance and high reliability required for the applications of laminate for electrical insulation and printed wiring board without using halogen elements, it is necessary to use an epoxy resin having a stiff structure, such as a phenolic novolak skeleton. For solving these problems, Japanese Unexamined Patent Publication No. 60951/1999 discloses a method in which the surface of inorganic filler is treated with a silicone oligomer to enhance the adhesion between the resin and the filler, whereby improving the dispersion property of the resultant mixture. However, such a surface treatment for the filler causes the production cost to rise, and is therefore difficult to apply to general-purpose laminates and prepregs.